This invention relates to horizontal, vertical or angularly disposed diaphragms which resist structural forces. As herein used, a diaphragm is a large, thin structural element that is loaded in its plane. A diaphragm at its most basic level has three components: a shear resisting element, which consists of one or more structural panels, a frame and a fastening system for connecting the frame to the shear resisting element. The structural panels which make up the shear resisting element are commonly made of 4xe2x80x2xc3x978xe2x80x2 plywood or Oriented Strand Board (OSB) sheets. Vertical diaphragms in a structure are commonly called shearwalls. Specifically, this invention relates to an improved method of constructing diaphragms. The present invention improves on standard construction and fabrication methods to reduce the bending of the fasteners that make the mechanical connection between the structural panels and the perimeter framing members that support the structural panels.
All structures must be designed to resist lateral forces. Structural units designed to resist lateral forces are commonly called lateral force resisting systems. Lateral forces on a building are typically produced by wind loading and seismic forces. Both, but especially seismic forces, cause cyclic loading, that is, the force on the building reverses direction. The extensive damage caused to buildings by the January 1994 earthquake at Northridge, Calif. has demonstrated that lateral force resisting systems must be improved to better resist cyclic (reversed) lateral loading.
In general there are three types of lateral force resisting systems used in framed buildings. The first type, rigid frames, resist lateral forces by bending in the frame members. The second type, trusses or braced frames, resist lateral forces by primarily carrying the resulting tension and compression forces in diagonal members or cross braces. The third type, shearwalls or diaphragms, are large, flat structural units that act like deep, thin beams with the structural panel or panels of the diaphragms acting as the xe2x80x9cwebxe2x80x9d of the beam and the chords of the diaphragms acting like the xe2x80x9cflangesxe2x80x9d of the beam. It is thought that shear deformation is the significant action in diaphragms.
The present invention provides an improved method of constructing shearwalls and horizontal diaphragms to serve as improved lateral force resisting systems.
When considered in its function of resisting lateral forces, a typical shearwall or diaphragm consists of three structural elements: a frame, a shear resisting element, and a fastening system for attaching the shear resisting element to the frame.
The diaphragm, in turn, is integrated into the structure via a connection system. The connection system must be designed to transfer the lateral forces imposed on the building into the diaphragm. In the case of shearwalls or vertical diaphragms, special anchors or transfer members for resisting the moment forces imposed on the shearwall may also be required.
In a shearwall, these special anchors typically consist of anchor bolts and holdowns, connecting the bottom of the chords to a structural member below the shearwall. These anchors resist tension forces acting to overturn the shearwall. A holdown is typically attached to the inner face of each chord with heavy screws, nails or bolts. The holdown receives a bolt which is connected to an anchoring structural member below.
Diaphragms and shearwalls are connected to the other structures of the building in a variety of ways so that lateral forces imposed on the building will be transferred to them. For example, it is common to attach a first story shearwall to the foundation on which it sits with foundation anchor bolts. The anchor bolts are embedded in the foundation and run through the bottom strut or mudsill of the shearwall and attach with a washer and nut.
When viewed in terms of resisting lateral forces, the frame is primarily an intermediate member, transmitting the lateral forces imposed on the building to the shear resisting element. It does this through the fastening system. In most diaphragms, the structural panels are attached to the frame with mechanical fasteners such as nails, screws or staples, spaced around the perimeter of the structural panels according to prescribed schedules. As used in this application, these fasteners located at the perimeter of the structural panel will be called perimeter fasteners to distinguish them from other fasteners located farther in from the edge faces of the structural panels. It should be noted that perimeter fasteners refer to the nails at the perimeter of the structural panels and not just the perimeter of the shear resisting element which can comprise a plurality of structural panels.
Nails commonly serve as the perimeter fasteners when wooden framing members are used. The perimeter fasteners are driven into the distal face of the structural panel at its perimeter, through the structural panel, and into the framing members. Through testing, the inventors have found that with improvements in the other elements of the typical shearwall, the perimeter fasteners have become the critical weak link through which failure of the overall system occurs.
The shear resisting element, as its name implies, works primarily in shear. The shear resisting element can be a single structural panel, if the diaphragm is small, or a number of structural panels, if the diaphragm is large. Typically structural panels for use in diaphragms are made of plywood or Oriented Strand Board (OSB) of structural grades, because they give a diaphragm high shear resisting values as well as having other desirable characteristics. Plywood and OSB come in many different grades. Typically, structural grades such as {fraction (15/32)}xe2x80x3 APA Structural 1 Rated Sheeting {fraction (32/16)}, Exposure 1 are used in diaphragms to obtain sufficiently high shear values. Other types of structural panels include: fiberboard, waferboard, particle board, gypsum wall board and high density particle board. Structural panels made from composites of different materials are also known in the art. U.S. Pat. No. 4,016,697, granted to Ericson on Apr. 12, 1977 teaches completely cladding one side of gypsum wall board with a thin sheet of steel to improve its structural characteristics. Kevlar is also beginning to be used with engineered wood products as ply materials.
The most basic frame consists of chords and struts located at the perimeter of the diaphragm. In a shearwall, the top strut is commonly called the top plate and the bottom strut is commonly called the bottom plate or mudsill. The chords are commonly called end studs. The framing members can be made of wood, engineered wood products, such as glulam, or steel, to name a few common materials.
Most diaphragms will be made with a variety of layouts of the framing members. If a diaphragm consists of more than one structural panel, framing members need to be placed at the joint or joints of the structural panels to tie them together, and provide support to the structural panels. If the diaphragm serves as a load bearing structural element as well as a lateral force resisting element, intermediate members will often be added to strengthen the diaphragm against particular forces.
A shearwall, for example, is typically designed to serve as a load bearing unit for the structures above it, as well as a lateral force resisting element. Both forces work in the plane of the shearwall. In a wood frame building, the walls are built with intermediate studs that connect the top plate to the bottom plate to give the wall sufficient load bearing capacity. Ordinarily, these intermediate studs are spaced 16 inches on center from each other and from the end chords to give the shear wall sufficient strength.
For example, in an 8xe2x80x2xc3x978xe2x80x2 shear wall, comprising two 4xe2x80x2xc3x978xe2x80x2 structural panels disposed vertically, intermediate studs spaced 16xe2x80x3 on center will occur both at the vertical joint of the 4xe2x80x2xc3x978xe2x80x2 structural panels and at intermediate points along each of the structural panels. The intermediate stud at the joint between the structural panels acts as a framing member to connect the structural panels to form a single shear resisting element. It also provides bearing support for the top plate. The other intermediate studs provide bearing support to the top plate. These other intermediate studs are also attached to the structural panels.
Sometimes, blocking members will be added to shear walls. Blocking members are relatively short lengths of board placed normal to and between the intermediate studs. If the shear wall is made with a horizontal joint between the structural panels, blocking will be added at the joint to connect the structural panels along the entire length of the joint. For shearwall action all edges of the structural panels must be supported. The blocking also provides support to the studs against buckling under compression loads transmitted through the top plate.
Horizontal diaphragms are also designed to serve as load bearing units, as well as a lateral force resisting systems. Horizontal diaphragms in wood frame construction serve as roofs and floor systems. Horizontal diaphragms, in their load bearing capacity, are loaded normal to their plane. As roof and floor systems, they are generally rather large; thus these diaphragms typically comprise a number of structural panels. These structural panels need to be connected at all their joints as well as the outer edges of the diaphragm. The structural panels also need to be supported at intermediate points to prevent excessive deflection under loading normal to the diaphragm. In a roof or floor, the framing members are given various names such as: stiffeners, rafters, joists, purlins, subpurlins, beams, headers, and girders. Blocking may also be provided.
As mentioned above, the inventors tested shearwalls made according to current building practices with the best commercially available components under cyclic loading conditions. They discovered that the predominant failure mode is the flexing and fatiguing of the nails around the perimeter or outer edges of the structural panels that connect the structural panels to the frame of the diaphragm. The present invention addresses this problem, allowing shearwalls and diaphragms to be both stronger and stiffer.
It is an object of the present invention to create a diaphragm that is highly resistant to failure under cyclic loading by lateral forces.
It is a further object of this invention to improve the mechanical connection between the structural panel and framing members of a diaphragm, so that the fasteners that make the mechanical connection can better withstand cyclic or reversing lateral forces by bending less under such forces.